wifi: mac80211_hwsim: fix link change handling

The code for determining which links to update in wmediumd
or virtio was wrong, fix it to remove the deflink only if
there were no old links, and also add the deflink if there
are no other new links.

Fixes: c204d9df ("wifi: mac80211_hwsim: handle links for wmediumd/virtio")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>